1. Field of the Invention
This invention relates to disk drives and, more particularly, to apparatus and methods for improving the seeking and tracking capabilities of a servo system for positioning a read/write head with respect to a track on the disk drive.
2. Prior Art
A disk file is an information storage devices which use one or more rotatable disks, with each disk having a number of concentric data tracks formed thereon for magnetically recording data. Each track includes a number of sectors. In a disk file which has less than four disks, it is necessary to use a digital sector servo control system to maintain a read/write head precisely over a desired track during a read or write operations. This is accomplished by utilizing prerecorded servo information, which is angularly spaced and interspersed among the data information on each of the disk surfaces of a disk. The operation of moving the head to a desired track is called seeking. The operation of maintaining the head over the centerline of a particular track is called track following. During both seeking and track-following operations, the prerecorded servo information is sensed by the head and demodulated to generate a digital gray code and a position error signal (PES). The digital gray code includes track identification information and the PES indicates the position of the head away from the centerline of a track. The digital gray code and the PES are combined together to generate a measured position signal. The measured position signal is then used in a servo feedback loop to generate a control signal to move the head back to the centerline of the target track. A description of a general digital disk-file servo control system is given by a U.S. Pat. No. 4,679,103 granted to Michael I. Workman and titled "Digital Servo Control System For a Data Recording Disk File".
There are several reasons for the position of a read/write head to be in error, or off track, during a track following operation. One of the major components of head position error is called the repeatable run out (RRO) at the disk rotating frequency which is an error caused by an unbalanced spindle or by a non ideal bearing. The Workman U.S. Pat. No. 4,679,103 does not specifically deal with this problem. The servo control system disclosed in the Workman U.S. Pat. No. 4,679,103 does not have sufficient gain at the run out frequency to fully correct for the RRO error.
Another U.S. Pat. No. 4,616,276 also granted to Michael I. Workman and titled "Digital Servo Control System With Fast Reduction Of Repeatable Head Position Error" deals with this problem. The Workman U.S. Pat. No. 4,616,276 uses a least mean square technique to adjust tap weights for every servo sample based on the measured PES signal. The Workman U.S. Pat. No. 4,616,276 then calculates a feed forward signal using the adjusted tap weights and the values of sine and cosine functions with a frequency which is the same as the disk rotational frequency. The feed forward signal is then added into a control signal to generate a modified control signal which causes the head to follow the track center line. A disadvantage of the Workman U.S. Pat. No. 4,616,276 scheme is that it adds more random noise into the PES signal during track following. Additional quantization noise is generated from additional multiplication operations required in the tap-weight and the feed-forward calculations. The magnitude of the additional quantization noise is approximately 1% to 2% of the track. FIG. 5 of the Workman U.S. Pat. No. 4,616,276 shows that it takes more than one disk revolution to remove the effect of the run out error.
An article entitled "Learning and Compensating for Repeatable Run Out of a Disk Drive Servo using A Recurrent Neutral Network" by S. Weerasooriya, published in 1993 by Magnetic Technology Centre in Singapore, describes a recurrent neural network which can be adaptively trained to generate a correct feed forward signal for removing the effect of the RRO error. This technique also takes more than 1 revolution for adaptation and requires nine tap-weights which generates even more quantization noise than the scheme described in the Workman U.S. Pat. No. 4,616,276. Both the Workman U.S. Pat. No. 4,616,276 and the Weerasooriya article use signal processing techniques to generate a feed forward signal for removing the RRO error. In general, a feed forward approach is more sensitive to variation than is a closed-loop servo solution for correction of RRO error.
A U.S. Pat. NO. 5,241,433 granted to Anderson et al. and titled "Disk Drive Servo Control" discloses a servo control system which includes a reference track in the outer guard bank of a dedicated servo surface and on each of the data surfaces. It combines a supplement PES, which is signal indicating the difference between the servo and the data surface, with a dedicated surface PES to yield a composite PES. The composite PES more accurately follows the track center of a disk surface being addressed. This technique does not remove the RRO error. It only removes the differential RRO which is the difference between the dedicated servo surface and the data surface.